a) Field of the Invention
The present invention concerns a copolymer of ethylene oxide and at least one substituted oxirane carrying a cross-linkable function, a process for the preparation thereof and the use thereof for producing a solid electrolyte having good mechanical properties, a good cationic conductivity and a good chemical compatibility with the electrodes of a generator which operates with alkali metals, such as lithium and sodium.
b) Description of Prior Art
Solvating polymers are known to be useful for preparing ionically conductive materials. Polymers of ethylene oxide or of dioxolane are polymers which solvate cations, in particular alkali cations such as for example the ion Li+ which is present in rechargeable electrochemical generators of the polymer electrolyte lithium battery type. However, these polymers are semi-crystalline, since crystallinity varies as a function of the molecular weight of the polymer. This semi-crystalline character of the polymers results in a decrease of the conductivity of the materials in which the polymers are present.
It has been found that it was possible to decrease the crystallinity of semi-crystalline polymers, without affecting their solvating properties and their electrochemical stability, by introducing defects in the macromolecular chain at possibly irregular intervals. However, it has been observed that the introduction in a semi-crystalline polymer, such as for example a high molecular weight, polyoxyethylene (POE), of units producing disparities, i.e. substituting a semi-crystalline polymer with a copolymer or a polycondensate, is frequently accompanied by a decrease of the molecular weights and a lowering of the mechanical properties, for example at high temperature. An attempt was made to overcome this disadvantage by introducing into the polymer, units which contribute to the formation of tri-dimensional networks by cross-linking the copolymer, before or after its formation. Because of the restraints imposed by the requirements of electrochemical stability, the particularly preferred units permitting cross-linking are selected among those which contain an unsaturated carbon/carbon bond, such as an allyl bond or a vinyl bond. The introduction of such units into a copolymer, additionally makes it possible to fix various groups, for example ionic groups, on the macromolecular chain.
Using an initiator based on organometallic derivatives of non-alkali and non-alkali-earth metals, for example an alkyl aluminum or an alkyl zinc, it is possible to prepare copolymers of an ethylene oxide and of an oxirane which carries an unsaturated substituent by coordination polymerization. See, for example, E. J. Vandenberg, Journal of Polymer Science, Part A-1, Vol. 7, pages 525-567 (1969).
This type of polymerization is not affected by the presence of a small amount of impurities. However, the reactivity of the various comonomers depends on their steric hindrance. Thus, in the production of a copolymer of ethylene oxide and an oxirane carrying a saturated substituent (for example propylene oxide) or an oxirane carrying an unsaturated substituent (for example allyl glycidyl ether), the polymerization yield of ethylene oxide is near 100%, while the yield of the substituted oxirane in a copolymer having a molecular weight higher than 1000 is only 60%. In addition, ethylene oxide is consumed preferably at the start of the polymerization. Because of the difference of reactivity of the monomers, the copolymer formed at the start of the polymerization contains an excess of ethylene oxide and has a higher molecular weight than the one formed during or towards the end of the polymerization reaction. The copolymer formed by coordination polymerization thus has long poly(oxyethylene) sequences which are crystalline and wherein the molecular weights of these sequences are highly heterogeneous.
It is also possible to polymerize saturated oxiranes such as ethylene oxide or propylene oxide through an anionic process. When such a polymerization is carried out with initiators of the sodium hydroxide or potassium hydroxide type in an aqueous solution or in protic solvents such as ethylene glycol, a number of transfer reactions towards the solvent take place, and the molecular weights obtained are very low. When the anionic polymerization of oxiranes is carried out in the presence of initiators of the potassium alcoholate or cesium alcoholate type in an aprotic solvent which solvates cations, or in the presence of complexing agents such as crown-ethers, ethylene oxide undergoes a living polymerization, i.e. the number average degree of polymerization (DPn) increases with the conversion rate, the distribution of molecular weights is narrow, the polydispersity I=Mw/Mn is near 1 and there is practically no transfer and termination reactions. An anionic polymerization which is carried out under these conditions enables one to obtain high molecular weights when the polymer is ethylene oxide. However, when used with monomers of the substituted oxirane type, it has only been possible to obtain low molecular weights (Mw<20,000) to this date. For example, the polymerization of styrene oxide initiated with potassium tert-butanolate gives a poly(oxystyrene) having a molar weight of 1000 g, and the growth of the poly(oxypropylene) chains is interrupted by transfer reactions towards the monomer [D-M Simons and J. J. Verbane, J. Polym. Sc. 1960, 44, 303]. When the monomer is phenyl glycidyl ether, chain growth is also rapidly interrupted by transfer towards the monomer [C. C. Price, Y. Atarachi, R. Yamamoto, J. Poly. Sci. PartAl, 1969, 7, 569]. In spite of the advantages associated with nearly quantitative conversion rates in the case of anionic polymerizations, the prior art shows a living character of the polymerization for ethylene oxide only.
The anionic polymerization of cyclic ethers such as ethylene oxide and its substituted derivatives (propylene oxide, butylene oxide, allyl glycidyl ether, etc.) requires a very low level of impurities such as water alcohol, etc. Copolymerizatiohn is slow relative to the homopolymerization of ethylene oxide, and requires therefore a much smaller amount of impurities in order to avoid the chain transfers and chain terminations which yield a low molecular weight copolymer (Mw less than 20,000 for example). To obtain a high molecular weight copolymer (Mw larger than 50,000, for example), one normally requires drastic purification techniques, such as drying and distillation on a vacuum line. Such extreme purification methods are suitable for the preparation of small amounts (1 to 20 g) of model compounds, but would be too expensive for the production of industrial quantities (over 100 kg).
A review of the prior art indicates that an electrolyte may consist of a copolymer of ethylene oxide, methyl glycidyl ether and a small amount of allyl glycidyl ether, the copolymer of course including an ionizable salt, all as disclosed in Couput U.S. Pat. No. 5,086,351. Similar copolymers are also disclosed in U.S. Pat. No. 5,206,756 (Cheshire); U.S. Pat. No. 4,578,326 (Armand); U.S. Pat. No. 5,350,646 (Armand); and French Patent 2 570 224. However, these copolymers have all been prepared by coordination copolymerization and thus have defective mechanical properties, as mentioned above. For example, a simple calculation with respect to the copolymer described by Cheshire in U.S. Pat. No. 5,206,756, column 25, will show that its polydispersity is 95. The other copolymers of the prior art, produced by coordination polymerization, all possess polydispersisty values which are higher than desirable to produce electrolytes with excellent mechanical properties.
The present invention aims at providing a copolymer of ethylene oxide and of at least one substituted oxirane carrying a cross-linkable reactive function, by a free radical process, which gives an ionically conductive material of improved mechanical properties as compared to the materials obtained from known copolymers of the poly(oxyalkylene) type, without decreasing the ionic conductivity by providing too many cross-linking points which would cause an increase of the glass transition temperature Tg, said ionically conductive material additionally showing an excellent chemical compatibility with the electrodes of a generator when the material is used as an electrolyte.